Load metadata when accessing a playlist page

[smlz]

Currently all metadata (index.json) is loaded once at the beginning of the application life cycle.

I did this because I think (thought) that transferring, parsing (JSON), and loading the rather large metadata file in a Vue.js app is slow. Too slow doing for it every time, the user wants to open another playlist. This was possible, because the metadata did not really change all that much in the first version off the app. Changes came only from playlist edits, which is usually done by a single person. So unless this person is using multiple browsers for doing the edits, the playlist metadata does not really diverge.

In the meantime we added the play_count and last_play items, that get updated regularly. So now, the metadata is always diverging, which now is a problem.

Also, I never bothered to actually check, what part of loading the metadata (transfer, paring, or loading) is actually the main bottle neck...

This issue is related to #280.

[smlz]

Measuring loading metadata:

• Transfer and JSON parsing: ~500 ms
• Search indexing: ~200ms
• Displaying: at least one second